Photocell multiplier apparatus



NOV. 25, 1947. w, SAUNGER 2,431,510

PHOTOCELL MULTIPLIER APPARATUS Filed Sept. 29, 1944 AMPLIFIER AMPLIFIER FIG.3

9 a 4 V 2 9 4 9 9 2 a v 5 9 9 a a a a 4 a 2 5 11 I ATTORNEY Patented Nov. 25, 1947 PHOTOCELL MULTIPLIEE APPARATUS Hans -W; G.- Salinger, Fort Wayne, Ind., a ssigno r,-}

b'ymesne assignmentsgto Farnsworth-Besearch Corporation, a corporation of Indiana Application September 29, 1944, Se1"ial NO 55 633 1 1 7 Claims.: (Cl. 250'-41;5)

a! "1% hidin a source isinconvenient andsometimes absolutely This invention" relates to target-locatin methods and devices; andparticularly to systems of this character employing photocell multiplier tubes."

According' to"conventional practice photocell multiplier tubes ofvarioustypes'havebeen used toprovide information regarding thevlocation of aremote targetirr'the'form, for'exa'mple, of a point of "light." Gen'erally,'the tubes'pre'viously" used'have'beenpf' two'classes. Onetype of tube is provided with a photoelectric cathode divided into separate areas of "predetermined form such as quadrants 'of a circular area and'has arranged cooperativelytherewith an individual electron multiplierfof'each subdivision of the cathode area; The other type of tube employs a solid photoelectric cathodand aplurality of electron niultipiiersihaving" anodestructure of suitable configuration to divide the. electron image as desired:

As is well known in the art; however, tubes of these types are capable'onlyiof providing informationregarding the general location of the target. These. tubes are incapable of supplyingdetailed information regarding the accurate location .of the target wi'th respect to a line of sight-'which conveniently may, be arranged I to coincide with' the optical axisof the tube.

all' devices of. this character employing photosensitive cathodes-there always is present a so-. called.-idark current. This is produced by a relatively small electronemissionfrom the oathode surface even in theabsence of lightp Where a it. is desiredto-use a-device'of this character for the-locationwof a point source of light, for-example, in: an: otherwise completely dark-image, it, therefore; is-seenthat-there is difiiculty in segregating/the photoelectric-currents resulting from. the emissionwhich'occurs even in the ab-- sence of light andthat produced by the projection of annoptical image of the-lightsource upon the cathode. Theseparation ot-these two currents, where the apparatus is-operated as a direct current device, is particularly difiicult: if itisdesired" to i obtaininformation with vany.v great degree of accuracy, especiallywhere the light from the target source is weak; In a system 'in whichthe light received from the target source is impulsive or iswinterrupted at *a-relatively. rapid rate, the apparatus a may: be operated as an alternatingcurrent :device and1the separation of the intelli-' comparatively L easy; matter. I many-cases the interruption 'of 'the light at the impossible: I

It is an objectorthe'present invention, therefore, to 'provide'a'novel target-seeking device in- .1' eluding a photocell tube for converting an image of the area including atarget into electrical intelligence representative of the accurate location of the target with respect to a predetermined point in'an' optical systemused in conjunctionwith the'tube: I

Another object of the invention is to provide a novel method of locating a target, whereby to develop electricareifects representative of the target location:

The apparatus in accordance with the present invention comprises a photocell multiplier tube havinga photosensitive cathode which may be, but not'necessarily, divided into a plurality of separate electron emissive areas. An individual electron multiplier 'rnay be provided in association with each ofthesegrnented cathode areas. The optical system with which such a device is provided includes means for projecting an optical image upon the cathode of the target light source in a manner to produce a substantially circular trace The'projecting means may include a lens" be moun'tedbetween the cathode and the light source and should be adapted for relative movement in order-to'interrupt the light.

The method' oflocatin'g the light target in accordancewith this invention comprises, efiecting-a rotating projection upon a photosensitive cathode of a light spot representative of a target source. The light projection upon the cathode is interrupted at a predetermined frequency and the alternating cornponent of the photoelectric emis'sionfrom-the 'cathode resulting from the two'forego'i'ng steps is amplified so that it may be'utilized to provide information indicative of together with other andfurther objects thereof,

referenceis'had to' the following description, taken 1 in connection with the accompanying drawing, and its 'scope will'be pointed out in the appended claims.-

This interrupting in the accompanying drawing:

Fig. 1 is a diagrammatic representation of apparatus embodying the present invention;

2 is a sectional view taken on the line 2-2 of Fig. 1, and illustrating one configuration of the photosensitive cathode, together with an associated electron multiplier arrangement in an embodiment of the invention in a preferred form;

Fig. 3 is a sectional view taken on the line 33 of Fig. 1, and illustrating the configuration of the lens supporting structure; and

Fig. 4 is a sectional view taken on the line l 1 of Fig. 1, and illustrating the structure of one of the light interrupting members.

Having reference now to Fig. 1 of the drawing, the apparatus embodying the present invention includes a photocell multiplier tube, the electrodes of which are mounted within an evacuated envelope H. In the illustrative form of the invention there is provided a photosensitive cathode [2 located adjacent one end of the tube envelope in such a manner that its sensitized surface faces the end of the tube. In this form of the invention the cathode comprises four substantially equal quadrant areas designated by the characters I, II, III and IV, and best illustrated in Fig. 2. Each of the quadrant areas of the cathode is distinct from the others. This arrangement may be provided in any conventional manner such as by setting separating strips between the quadrant areas or by cutting narrow slots in the cathode surface.

Associated with each of the quadrant areas of the cathode there is provided a multistage electron multiplier, the first stage electrode of each of which is located adjacent one segment of the sensitized surface of the cathode l2, For example, the multiplier associated with the cathode quadrant II has a first stage electrode l3 with the input opening thereof located adjacent to the peripheral edge of this cathode quadrant. In like manner, the first stage electrode M of the electron multiplier associated with the cathode quadrant IV is similarly located with respect to the peripheral edge of this portion of the cathode. Each of the multipliers may comprise as many secondary electron emissive electrodes, arranged in a conventional series manner as illustrated, or as otherwise desired, as may be needed to effect the required multiplication of the electron emission from the cathode. Each of the multipliers is provided adjacent the final secondary electron emissive electrode thereof with an electron collector electrode such as l5 and [6 respectively of the multipliers associated with the cathode quadrants II and IV.

As illustrated, each of the photocell. electron multiplier units is connected to a source of energy for the impression of the required operating potentials upon the component electrodes. In the case of the unit including the cathode quadrant II, the energy source may be a battery I! provided with a plurality of taps at appropriate points so that the desired voltages may be derived therefrom. Similarly, a battery It furnishes energy to the unit including the cathode quadrant IV. of employing separate sources of energy for each of the units comprising the apparatus embodying the invention, a single source may be employed with substantially equal facility. In such a case the connections of corresponding electrodes of the different units of the device may be made to the same points on the common energy source in a manner well known in the art. This latter type It is contemplated that, instead of connection may be made most conveniently by interconnecting the corresponding multiplying electrodes within the tube, so that only one set of conductors need to be extended to the energy source, In the present case the cathode quadrant II is connected to the negative terminal of the battery IT. The first stage multiplier electrode I3 and succeeding multiplier electrodes are connected to points of increasing voltage on the battery I! in a well known manner. The collector electrode I5 is connected through an output impedance such as a resistor [9 to the positive grounded terminal of the battery H. In like manner the cathode quadrant IV is connected to the negative terminal of a battery l8 and the collector electrode I6 is connected through an output impedance such as a resistor 20 to the grounded positive terminal of this battery. In case a single energy source is used for the multiplying electrodes, it nevertheless will be necessary to separately connect the collector electrodes to their respective output impedances.

The alternating current components of the signal voltages developed in the output resistors l9 and 20, in a manner to be described, are impressed upon individual alternating current amplifiers 2i and 22, respectively. Condensers 23 and 24 provide the necessary alternating current coupling between the respective output resistors l9 and 20 and their associated amplifiers 2| and 22.

In order to project light coming from a distant target source P located at the left of the apparatus as shown in the drawing, there is provided an optical system which includes a condensing lens 25. This lens is supported in a circular disk 26 in such a manner that the lens is located eccentrically with respect to the axis through the center of the supporting disk. Also, rigidly mounted to the lens supporting disk 26 is another disk 21 which is provided with alternate transparent and opaque sectors. The configuration of the disk 21 and its relationship to the lens 26 is best illustrated in Fig. 3. This disk is provided within the area of the lens with a plurality of spaced transparent sectors such as 28 with which are alternated a plurality of spaced opaque sectors such as 29. These sectors radiate from the center of the disk. All portions of the disk outside of the lens area are opaque, as illustrated. The unitary structure comprising the two disks 26 and 21 and the lens 25 is arranged for rotation about the central axes of the two disks by any conventional means (not shown). In the present form of the invention, it is preferred to arrange the axis of rotation of the lens structure to coincide with the central axis of the tube through the center of the quadrant cathode [2.

There also is provided a disk 30 which may be located between the rotating lens structure and the tube cathode l2. The disk 30 is a stationary structure and is located coaxially with the disks 26 and 21. It also comprises a plurality of alternate transparent sectors 3| and opaque sectors 32 in like numbers and arrangement with respect to the center of the disk as those of the disk 21. In this case, however, the sectors extend to the peripheral edge of the disk 36, as illustrated.

Referring now to the operation of the described apparatus, assume that light coming'froma dis-" tant target P is to be accurately located with re spect to the central axis of the photocell multiplier tube. In the assumed instance, the target P is located somewhat above the line extendcaesium projected as a r'elatively small moving spot of light -onto the photoelectric" cathode [2. By reasonof the eccentrically mounted lens- 25, thepath of the light spot on thecathode' is circulan'as indicated 'bythebroken line 33 of Fig. 2. The center of T the "circular path 33 4 on the cathode represents the =locat-ion of the target light source Pwithrespect'to the tube axis. Therm-ct course, is not any" visible indication of thisactuar target location on-the' cathode. However; for the purpose "of clar ifying this description the target" location on the calihddel indicated-- iri'FigL-Q by the point flvwhichpit is seenplies at a"small distance directly "below the center of the cathode in the iquadrant IV. It-lies below" rather than above the cathodecenter by reason of the image inverting property of the: 1ens25.

The light which is projected onto the cathode I2 is I interrupted at a 'predetermined rate by means of the continuousl changingrelationshi of the" disks 2] -and '3ll. When the transparent sectors: of these two disks such askzs and '3 i respectively "are in register, light from I the source P is-projected onto -s'om'e 'por'tion of thecathode. At' an instant later "when the trans arent 1 sectors "s'u'ch as 28 of thedisk fl are in register with the opaque sectors such'assz Of the diSk'Sm-the light *be'a mis interrupted and does n'ot' reach the cathode. It may beseenthat the provisionof a plurality of suchalternate transparent and opaque sectorson the disks'2l'an-d 30"willproduce an interruption of the light beam at a rate determined by'the' number-of disk sectorsand the speed of rotation of the disk fl relative tothe fixed dis'k 3B. The resultthen is a succession of light impulses "pro jectedpntothe cathode along a circularpathsomewhat in the manner indicated by the' brokendine' 33 of Fig. 2.

The electron emission fro'm the'catho-de quadrant'II, for example; may be amplifiedto any desired degree by the associated electron multiplier which includes the first stage' -ele'ctrode l3. The electron emission from this and ii all other cathode quardants "comprises a'steady emission commonly called the dark current; plus an intermittent emission of somewhat greater intensity resulting from the light impulses. projected onto this cathode quadrant from the target light source. 'There thus is' developed "in the output resistors such as 19 asignalvoltage which'has a steady direct current "value representative of the SO-called dark currentcathode" emission and, in addition;- asuperimposed pulsating voltage which represents the'alternating current component indicative of the location of the target light/source with respect to that particular cathode quadrant. By reason of the employment of the coupling condensers such 'as -23,' b'etween the respective=output resistors I and the associated amplifiers, there are impressed upon theseamplifiers only the -so-called alternating current'components of thevoltages developed in the output resistors. -In-this manner it is-seen that the socalled dark current emission represented by-the direct current component of the voltages developed inthe outputresistors is eliminated from the signals developed in the output circuits'of the amplifiers. Obviously,.the multiplier output impedances, herein representedbyresistors 19 and 20, mayinstead comprise antiresonant circuit components tuned -for I response substantially d only at the light interruption frequency. In sucha case the coupling condensers may be eliminated.

As indicated"previously,. a device of this character' will be providedwith an amplifier for each section 1 of the-cathode. Inthe embodiment of the inventioncho'sen for illustration herein, a device having a quardant cathode will beprovided with iour'amplifiers similar-to the amplifiers I 2 I a-nd'22. T-he average eutput signal from any'amplifier over a -period of time is representative of the length ofthe arc of the circular trace #3! made by the 'projection of-the light spot on the cathode quadrant associated with that amplifier. Obviously, the amplifier output voltage repre- H sentative of the length of the arcof the trace 33 0m- 9, given cathode quadrant, when considered in relationto the voltages representative of the'other arc lengths-of the circular trace on the other cathode quadrants, is information which can be correlated, in a manner which subsequently will be descr'ibed, to indicatethe'position of the target light: source'w ith respect to the center of the cathode. The cathodecentenior convenien'ce, is'a'ssumed to lie on the central axis of the tube.

Asan example, consider the signal voltages which will be developedunder the assumed conditions in the respective output circuits of amplifiers 2| and 22 as representative of the portions 'of the circular trace 33 projected onto the respective cathode quadrants II and IV. If the target light source'is located as 'assumed in the position indicated by thei point 34, it is obvious that the arc-length of the trace 33 appearing in cathodequadrant' IV- willbe considerably greater than that appearing -in' cathode quadrant II. Considering the periodic interruption of the light beam by the -lens supporting disk 21 and the cooperatingstationary disk 30, assume that, in the time the lighV-projecting optical system'is functioning to direct light onto the cathode quadrant II, thereoccurinterruptions of the light beam resulting in the projection onto this cathode quadrant of ten light impulses. In like manner, assume that in cathodequadrant IV there are projected thirty such light impulses. Inasmuch as all-of-the light impulses are of equal duration and intensity, it follows that the total emission'resulting from the light projection onto the cathode quadrant IV will be substantially three times greaterthan the total emission from the cathode quadrant II. It'will be understood that the electron emission from the cathode areas referred to is that resulting from "the projection of the optical image of the target light source onto the cathode and is in addition to the so-called dark-current emission.

After suitable multiplication by means of the described electron multipliers associated with each of the cathode quadrants, it is seen that the alternating current components of the signa1 voltages developed in the output resistors l9 and Zllwill be 50 related that the energy impressed upon theamplifier 22 will be substantially three times. greater than the energy impressed upon the amplifier 2|. Similar conditions, of course, will obtain in the respective output circuits of these amplifiers.

In -substantially the same manner, the amplifiers (not shown) associated with cathode quadrants I and III will produce in their respective output circuits signal voltages representative of the arc lengths 0f the circularly projected light path 33 appearing in -'-these quadrants. Therefore, the four amplifiers associated with the four quadrant areas of a photoelectric cathode will produce signal voltages from which may be derived, by any conventional means. information regarding the accurate location of the center point 34 of the circular light trace 3| relative to the center of the cathode. This information, of course, under the assumed conditions also will be representative of the accurate location of the target light source P relative to the central axis of the tube. As indicated, the means for utilizing the signal voltages derived from the amplifier may be conventional and for this reason is not shown specifically herein for the reason that itcomprises no part of the instant invention.

It followsftherefore, that the photocell multiplier tube maybe oriented or aimed in a manner to bring the axis thereof in substantial alignment with the target light source P. In such a case, it is obvious that the center point 34 of the light trace 33, which then will be projected onto the cathode I2, will coincide with the center of the cathode at the intersection of the co-ordinate axes thereof. In such a case, the signal voltages derived from the output circuits of the re spective amplifiers associated with the four cathode quadrants will be substantially equal. Therefore, these output voltages may be utilized in a manner to indicate that the circular light trace 3| is centered about the origin of a cathode axis.

It is contemplated to be within the scope of the instant invention that a photocell multiplier tube may be provided with a cathode divided into any desired number of segments, and not necessarily into quadrant areas, as illustrated. In such cases, the amplifiers associated with each of the cathode segments may be connected to suitable utilization circuits whereby the necessary comparisons between the respective output voltages may be made in order to produce the required information regarding the location of the target light source relative to a predetermined reference axis. Obviously, this reference axis need not necessarily coincide with the central or any other axis of the photocell multiplier tube. Also, it is considered that the instant invention is not limited necessarily to the employment of any particular type of electron multiplier. It is considered to be obvious that the electrode structure and arrangement of such multipliers may be substantially as desired to suit a particular type of tube. It also may be necessary in some applications to dispense entirely with electron multipliers used in cooperation with a photosensitive cathode. The emission from each cathode segment may, in such a case, be suificient for employment directly in the production of signal voltages in an associated output impedance device. The invention also is conidered to be of sufiicient scope to include the interposition of the light-interrupting disks between the target light source and the lens 25, instead of in the locations illustrated herein.

While there has been described what, at present, is considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a target seeking device, a photosensitive cathode, means including a rotating lens located in front of said cathode for projecting light from a target light source onto said cathode in a manner to produce a circular trace having a center located with respect to a predetermined point of said cathode representative of the target source location relative to said cathode point, and movable means located in front of said cathode for intermittently interrupting said projected light.

2. A target seeking device comprising, a photosensitive cathode divided into a plurality of electron emissive areas, means adjacent the respective emissive areas of said cathode for individually collecting the electron emission from each of said cathode areas, a lens located in front of said cathode for projecting light from a target light source onto said cathode and mounted for eccentric rotation relative to a predetermined point of said cathode, and rotating means located in front of said cathode for periodically interrupting said projected light.

3. A target seeking device comprising, a photosensitive cathode divided into a plurality of electron emissive areas, means adjacent the respective emissive areas of said cathode for individually amplifying the electron emission from each of said cathode areas, a pair of substantial-;

1y identical devices each having a plurality of alternate transparent and opaque areas mounted between a target light source and said cathode, said devices being movable relative to one another to interrupt light projected onto said cathode from said source, and a lens mounted between said source and said cathode and adapted for eccentric rotation relative to the center of said cathode.

4. A target seeking device comprising, a photosensitive cathode divided into a plurality of electron emissive areas, an electron multiplier individual to and operatively positioned with respect to each of said cathode areas, a pair of cooperatively arranged disks, each having a plurality of alternate transparent and opaque sectors mountedbetween a target light source and said cathode, said disks being rotatable relative to one another to interrupt the light projected onto said cathode from said source, and a lens mounted between said source and said cathode and adapted for eccentric rotation relative to the center of one of said disks.

5. A target seeking device comprising, a photosensitive cathode divided into quadrant electron emissive areas, a multistage electron multiplier individual to and operatively positioned with respect to each of said cathode areas, a pair of coaxially arranged disks each having a plurality of alternate transparent and opaque sectors mounted between a target light source and said cathode, one of said disks being stationary and the other of said disks being rotatable about its center, and a lens mounted eccentrically relative to the centers of said disks between said source and said cathode and adapted for rotation about the center of said disks.

6. A target seeking device comprising, a photosensitive cathode divided into quadrant electron emissive areas, a multistage electron multiplier individual to and operatively associated with each of said cathode areas, an alternating current amplifier capacitatively coupled to each of said multipliers, a pair of disks coaxially arranged With the center of said cathode and each having a plurality of alternate transparent and opaque sectors, said disks being mounted between a target light source and said cathode, one

of said disks being stationary and the other of said disks being rotatable about its center, and a lens mounted eccentrically relative to the centers of said disks between said source and said cathode and adapted for rotation by and with 5 said rotatable disk.

7. The method of locating a target light source which includes the steps of, projecting light from said source upon a photosensitive cathode in a substantially circular path, interrupting said 10 projected light at a predetermined frequency,

and separately amplifying the resulting electron emission from individual areas of said cathode.

HANS W. G. SALINGER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

